It is known to exploit the frequency shift of oscillating (vibrating) mechanical systems, in particular of micro- and/or nanomechanical systems, for the purpose of ultrasensitive, quantitative identification of small masses (e.g. within the range of several femtograms to nanograms).
Examples of possible applications of such mass-sensitive sensors are found in biological and chemical sensor technology. For this purpose, a mechanically oscillating membrane comprises a functional layer to which essentially only analytes of a specific type adhere and/or into which only analytes of a specific type are incorporated. Examples are, e.g., adhesion of antibodies to which only matching antigens may dock (key/lock principle). Systems known as oscillating systems are quartz crystal microbalances (QCM), micro- or nanomechanical beam structures (cantilevers) and also membrane structures, for example.
For example, utilization of functionalized so-called CMUT (capacitive micromachined ultrasonic transducer) structures has been suggested for identifying analytes. Such structures are essentially formed as parallel-plate capacitors having one fixed and one elastically movable electrode, respectively. By applying a periodic voltage between the two electrodes, the movable electrode may be excited to perform mechanical oscillations. If the exciting frequency is tuned in the vicinity of the mechanical resonance frequency, the deflection that is achieved accordingly is excessive. Such structures may be manufactured using known methods of surface micromechanics.
In the U.S. Pat. No. 7,305,883 by Khuri-Yakub et al., a mass-sensitive chemical sensor is described wherein the mass-sensitive elements are configured as CMOS structures. The mass-sensitive elements are electrically wired into the feedback path of an amplifying circuit, e.g. of the so-called Pierce type. The amplification and/or feedback factor is adjusted such that the oscillator starts to oscillate. In an advantageous embodiment, the CMOS sensor chip is provided, in accordance with conventional technology, with wafer vias and is bonded onto a wafer comprising readout electronics.
With this type of mass-sensitive sensors, which are operated as “self-oscillators” at the resonance, the change in the oscillation frequency is evaluated by additional loading with mass (adsorption, or absorption) on the part of the analytes, e.g. by means of a frequency count.
However, for a high level of identification sensitivity, such self-oscillating circuits necessitate a high quality, which may result in, e.g., low sensitivity when measuring dense media. Moreover, such circuits are sensitive to thermal drifting of structural components. In addition, a mass-sensitive switch has been suggested by M. Younis et al. in the article “Exploration of New Concepts for Mass Detection in Electrostatically-Actuated Phenomena”, J. Computational and nonlinear Dynamics, Vol. 4, 2009. Said switch is also based on an electrically excited micromechanical system; however, a non-linear electromechanical effect, i.e. the so-called “pull-in instability” or “escape” effect, is used here.
In the mass-sensitive switch proposed there, the parameters, i.e. the DC and/or AC amplitude(s) of the exciting voltage, are set such that the oscillating electromechanical element is pulled onto the fixed electrode (“pull-in effect”) within a narrow frequency range (instable frequency range) in the vicinity of the resonance. This may visually be described by mass escaping from a potential well in the event of too large an energy supply in the vicinity of the resonance. If the membrane is initially excited within a frequency range below the instable frequency range, applying an additional mass onto the oscillating electromechanical element will shift the resonance frequency such that the excitation frequency will now be within the instable range and that the “pull-in effect” will occur. The switch is mechanically closed. However, since this arrangement evaluates two states only, mass identification with the aid of the non-linear electromechanical “pull-in effect” is not quantitative.